The invention concerns a method for filling barrels, especially kegs, with liquids, in which at least one gas is dissolved, whereby the barrel is pre-stressed with a pre-stress gas before being filled with liquid, after which liquid is fed to the barrel by means of a filling valve of a filling station, connected to a feed line, and the pre-stress gas contained in the barrel is removed during the filling process, as well as a device to carry out this method.
Carbon dioxide-containing beverages such as beer only keep their CO.sub.2 in solution, if the partial pressure of the CO.sub.2 gas above the liquid is at least as high as the saturation pressure in the liquid. If the gas pressure above the liquid is below the saturation pressure, the liquid loses CO.sub.2 ; but if the gas pressure is substantially above this, there is a danger that additional CO.sub.2 will go into solution. The uptake of gas will depend on the differential pressure between the saturation pressure in the liquid and the partial pressure above the liquid, the time available for the gas exchange, which is generally the same as the filling time of the barrel, and the size of the gas exchange area, i.e., the liquid surface. The danger of a gas uptake during the filling is significantly increased because of turbulence in the liquid occurring during the filling process. However, the gas exchange between liquid and the overlying gas atmosphere concerns not only the CO.sub.2, but also other gases present in the gas atmosphere, especially oxygen, which is taken up by the liquid according to the same laws. Yet oxygen is a major factor of product quality in the case of liquids, which call be damaged by microorganisms or whose shelf life is endangered by oxidatior of the liquid's components.
In order to get the product through a valve into the barrel, whether a bottle or a keg, a differential pressure between feed line and interior of the barrel is necessary. The magnitude of the differential pressure determines the inflow rate of the product. Usually, in order to avoid increases of surface due to turbulence, the product is filled with initially low speed, which is then slowly increased. For this, the barrel is pre-stressed with a gas pressure that is substantially higher than the saturation pressure of the gas dissolved in the liquid. The actual liquid being filled is also maintained at this pressure level by tanks or pumps and supplied to the filling machine. After the pre-stressing of the barrel to the pressure of the supplied liquid, a connection is established between barrel and product feed line. The filling of the barrel with product is made possible by controlled venting of the pre-stress gas present in the barrel. In this process, the differential pressure which is built up determines the flow rate of the liquid. Moreover, it is known that the gas escape is throttled toward the end of the filling, which reduces the differential pressure between the interior of the barrel and the feed line. This has the effect of reducing the quantity of product filled per unit of time toward the end of the filling process, which enables a precise shutoff when reaching a set quantity. This known method is termed "back gas control". The advantage of this control method is that the gas pressure above the liquid is at all times above the saturation pressure of the CO.sub.2 gas.
The pre-stress pressure to be established is found by trial and error. At the start of the filling, the product should lose CO.sub.2 through turbulence, which results in local underpressures. This produces a desirable artificial foam on the liquid surface, whose bubbles contain only the released CO.sub.2 and, thus, protect the product against contact with the oxygen-containing gas atmosphere above it. During the further filling, the turbulence vanishes and so do the local underpressures. During the remaining fill time, the product again takes up CO.sub.2. Thus, the trick is to achieve an equilibrium between loss and further uptake of CO.sub.2 as a function of CO.sub.2 content, temperature, barrel size, and estimated filling time.
Apart from the fact that the barrel has to be pre-stressed far above the saturation pressure in the case of back gas control and venting has to be conducted in a controlled manner in order to achieve a controlled filling rate, the reduction in the filling rate in the last filling segment is a problem. At constant inlet pressure of the liquid, the flow velocity can only be reduced if the differential pressure is decreased. For this, in the known techniques, the gas escape is throttled (or shut off, in extreme cases) and one waits until the increasing liquid level has achieved a reduction of the counterpressure to the desired value by compressing the remaining gas volume present in the barrel. This period of time can be considerable, especially in the case of beer barrels. Thus, a 50-liter keg usually has an inlet cross section DN21 and a maximum filling rate of 3 l/sec. at a differential pressure of 0.8 bar. If the keg is filled with 35 liters, then 15 liters of gas space must be compressed by 0.7 bar to reduce the rate. This requires 15.times.0.7=10.5 liters of liquid and (given the reducing rate of filling) around 8 seconds of filling time. Thus, a fast, accurate control is not possible, especially with possible fluctuation in feed pressures. Even more critical is the situation when not just one gas (for example, CO.sub.2), but two gases (for example, CO.sub.2 and N.sub.2) are deliberately dissolved in the product. N.sub.2 is added to beer nowadays for its foam stabilizing action. The best example of this is stout beer, whose creamy, long-lasting foam is produced by the dissolved N.sub.2 released during tapping. But N.sub.2 and CO.sub.2 have completely different solubilities and saturation pressure curves. While CO.sub.2 goes easily into solution and can only be brought out of solution with difficulty, it is extremely hard to place N.sub.2 in solution at all, and very easy to take it out of solution with the smallest amount of turbulence. A balance between outgassing at the start of filling and recapture of lost gas during the filling is almost impossible to find in the case of 2-gas systems. The quality of the product being filled therefore fluctuates. One tries to compensate for this by maintaining the ratio of the gas atmosphere CO.sub.2 to N.sub.2 different than the proportion of the dissolved gases. But this compromise only holds for one temperature or one barrel size and only for one product feed pressure. Mastery of these many factors and their tolerances with a control technique is not possible. Another drawback of back gas control is that the barrel needs to be pre-stressed far above the saturation pressure with gas, generally CO.sub.2, in order to accomplish a pressure drop, which still lies above the saturation pressure of the gas even during maximum lowering of the interior pressure during the filling process. Since the gas is released into the atmosphere, the consequence is thus an increased consumption of the greenhouse gas CO.sub.2, in addition to the consumption of energy.
Thus the object of the invention is to make smooth filling possible and to reduce the consumption of pre-stressing gas.
In accordance with the invention, this object is solved, in essence, in that the pre-stressing gas in the barrel is pre-stressed merely to a partial pressure which corresponds approximately to the saturation pressure of one of the gases which is dissolved in the liquid, which is being introduced, whereby this partial pressure is below the product pressure which is present in the supply line in front of the filling valve.
In this connection, the pre-stressing of the barrel initially takes place as closely as possible to the product pressure at the filling valve, in order that injection of the product into the barrel shall be prevented when opening the filling valve. Instead of producing the pressure difference for the filling process by reducing the level of gas pressure in the barrel and keeping the product supply pressure constant, as in the case of return gas regulation, it is proposed according to the invention that the internal gas pressure in the barrel be kept constant and that the product supply pressure at the inlet of the barrel be increased, in order to produce the necessary pressure difference.
There are basically two possibilities in this regard for introducing the product into the supply line. This can take place either as is provided in the case of a first form of embodiment of the invention, by producing a pressure that is at or is released into the atmosphere, the consequence is thus an increased consumption of the greenhouse gas CO.sub.2 in addition to the consumption of energy.
A bottle-filling plant is known from U.S. Pat. No. 3,395,739 which operates in conjunction with a carbonization plant. A pump is installed behind the carbonization plant; in order to improve the solubility of the carbon dioxide, the pump increases the beverage pressure considerably above the saturation pressure of the liquid. A condenser (or a heating unit which briefly heats objects to high temperatures with a condenser which is connected with it) is connected to the pump, whereby the condenser will reduce the saturation pressure of the solution. However, a higher pressure than the saturation pressure will be maintained via a pressure maintaining device, whereby the liquid is supplied to the filling head at a pressure of 35 to 40 psi. Filling takes place via the valve in the filling head by means of a drop in pressure, whereby the pressure is reduced to approximately 10 psi before the liquid is introduced into the bottle which has been pre-stressed at approximately 6 psi. However, no form of regulation device is assigned to the valve of the filling head. This means that the filling pressure in front of the bottle is at a higher pressure than the pre-stressing pressure and that this pressure is decreased as a result of the pressure drop in the valve. However, the decrease in pressure is constant, so that the product pressure is constant both in front of the pressure-reduction valve and after the pressure-reduction valve. This means that no increase in pressure takes place during the filling process. Thus a controlled alteration of the filling velocity as a function of the level to which filling takes place in the barrel is impossible.